Various roller band devices of the type commonly known as Rolamites have been suggested for use in a variety of valving and force transmitting or translating applications. Such devices employ two or more rollers positioned between opposing guideways with a flexible band convoluted over the rollers and attached at its ends to the respective guideways. Such devices are characterized by low frictional forces between the moving members but they are also relatively large and the moving parts are heavy because of the weight of the rollers.
Another prior art type of flexible band device is described in U.S. Pat. No. 3,021,861 to Billeter et al. It employs two or more flexible bands which are rolled onto or off of valve ports to close and open the ports. This type of device is lighter in weight than the Rolamite device, and is widely used in air eliminators or fluid segregators. Inherently, this type of device is unbalanced in the longitudinal direction wherefor the force required to move the bands in one direction is different from that required to move the bands in the other direction. When used in an air eliminator, for example, the bands may be used as valve elements for opening and closing a plurality of ports but they cannot be used for opening one port while closing another port.